I bought a workhorse unintentionally this year--my Paul Green 'Jay' chelsea boots. They work with everything I wear and were a departure for me in the sense that I generally wear a higher heel. In any case, I wear them several times a week. They fit perfectly and seem to upgrade the look of whatever else I'm wearing.
For me a workhorse is a piece of clothing that I reach for again and again because it's versatile enough to work in lots of different situations. As kkards said, "i do think fit and function is a big part of what makes a workhorse."
My Eileen Fisher tank dress is a workhorse because it fits perfectly and works as a blank slate for whatever else I wear with it.
I have a black wool blend coat that I bought at Target several years ago that I wear almost daily in the winter. It's another item that fits perfectly. I have tried on multiple coats since I bought that one and nothing so far compares. It looks like it was tailor made for me.
My RM satchel is a workhorse because it seems the perfect size for everything I typically carry with me. It also keeps things fairly organized due to its structure. The grommet straps and exposed zipper detail (not visible in the find) on the edges add a bit of interest.
It took me years to find a denim jacket that I liked and eventually I ended up with two. In the spring and fall I wear them all the time. One is a bit oversized and a medium wash -- the other one is a dark wash and more fitted.
I don't think every item in my wardrobe has to be a workhorse. I have special occasion pieces that I love but since they're worn infrequently I don't consider them workhorses. On the other hand I wish I could predict with more assurance what will and won't become a workhorse.

So what is the difference between a workhorse and a basic then? Is it just that a workhorse is more noticeable/distinctive?

I have two workhorses that I acquired this summer, a red handbag and a white cape. I just seem to be able to wear these with everything and really enjoy using them. They add a bit of interest to any ensemble. The white cape jacket is an unexpected workhorse. I thought that it was too distinctive and may not work with other items, but really it can be worn the same as a basic white cropped jacket - so it has versatility.